1. Field of the Invention
The present invention relates to defect review systems, and ore particularly, to an apparatus and method for redetecting and classifying defects that exist on an object.
2. Description of the Invention
Microelectronic devices are typically fabricated, in part, by forming features (e.g., patterns) on selected layers of a semiconductor wafer. The escalating requirement for high density performance associated with ultra large scale integration (ULSI) semiconductor devices requires design features of sub 0.25 micron, increased transistor and circuit speeds, high reliability, and increased manufacturing throughput for competitiveness. The reduction of design features to 0.18 micron and under challenges the limitations of conventional semiconductor manufacturing techniques. Moreover, as design features are reduced into the deep sub-micron range, it becomes increasingly difficult to maintain or improve manufacturing throughput, (i.e., production yield) for competitiveness.
One factor that affects manufacturing throughput is the presence of defects on the semiconductor wafer during the manufacturing process. Defects can take various forms, such as, for example, scratches, particles, and unremoved portions of material layers on the surface of the semiconductor wafer. Undetected defects can often lead to failure of a semiconductor chip that is made from the wafer.
An in-line inspection and review is normally performed to detect and to classify defects that are detected on the semiconductor wafer during the manufacturing process. Classification of defects on the semiconductor wafer involves, among other things, the ability to extract accurate information such as defect size, shape, and boundary in order to identify the sources of the defects. This operation requires very high resolution imaging. As features on the semiconductor wafers become smaller, however, the size of the defects that can affect production yield falls below the resolution of conventional light optics. Therefore, the ability to classify defects using optical systems is becoming highly limited. Accordingly, there is an increasing need for higher resolution systems for defect classification.
The Scanning Electron Microscope (SEM) is capable of resolving features with a size of a few nanometers and, when combined with analytical tools such as Energy is Dispersive X-ray Spectrum (EDX), is a natural candidate for carrying out the defect classification on semiconductor wafers. Generally, an inspection system is used to scan the semiconductor wafer and generate a defect map of locations on the semiconductor wafer suspected of having defects thereupon. The defect map is then transferred to the SEM to acquire high-resolution images of each defect. The defect maps generated by inspection tools suffers from low accuracy, relative to the size of the defects. Hence, the SEM must xe2x80x9credetectxe2x80x9d (i.e., re-find) the defects, before generating the high resolution image required for classification. Specifically, the accuracy of the inspection tool is sufficient for detecting the presence of the defect, but insufficient for accurately determining the location of the defect. The defect map generated by the inspection tool is therefore unable to guide the SEM to the exact location of the defect. Accordingly, redetection functions as a bridge between the optical inspection tool""s output and the ability of the SEM to satisfy the demand for high resolution imaging of the defect for classification purposes. Of course, the smaller the defect, the smaller the field of view of the image and thus, a more accurate location of the defect must be known in order to redetect the defect. Moreover, when EDX or Auger analysis is performed, the required accuracy must be better than the defect""s size.
Unfortunately, due to conflicts between high sensitivity and fast operation of the optical inspection tool, defect maps of the optical inspection tools are not sufficiently accurate for fast redetection by the SEM. Specifically, systematic errors are introduced due to inaccuracy in various system components or inaccuracy in aligning the wafer. Even after minimizing the systematic error, there is a relatively large degree of uncertainty in the reported defect locations (i.e., defect map) due to the inspection is system""s settings. For example, in order to increase the throughput of the inspection system, the spot size used to scan the semiconductor wafer is normally selected to be much larger than the defects"" size. Thus, the reported coordinates of the spot size location encompass an area that is much larger than the location of the defect.
For example, uncorrected position error caused by settings of the spot size can be in the order of xc2x110xcexc for patterned wafers, and may exceed xc2x150xcexc for unpatterned wafers. The magnitude of this error unacceptably increases the search window size for the SEM in order to redetect the defect. For example, detection of a 0.2xcexc, defect in a field of view of 20xcexc results in an image of 5xc3x975 image pixels for the defect (at 500xc3x97500 pixels in image). This is a very severe requirement for SEM based detection systems due to low contrast to noise ratio generally achieved in SEM imaging. Consequently, while a reliable SEM redetection system, such as described in U.S. Pat. No. 5,659,172, may be used to find the defect, it can take an unacceptably long period of time, hence resulting in reduced throughput for the system.
Moreover, SEM-based redetection is ineffective for defects buried under (or within) an optically transparent layer. Consequently, it may be impossible to obtain a SEM image of the buried defect with sufficiently clear details to facilitate classification. Further, small variations in layer thickness are often reported as defects by inspection systems, but generally very difficult to redetect using SEM. Therefore, such a defect may not be classifiable using the SEM image.
Accordingly, one problem associated with current methods of reviewing defects on materials such as semiconductor wafers, is the inability for SEM-based review tools to quickly and accurately redetect the defects based on a defect map generated by an inspection tool. Another problem associated with current methods of reviewing defects is the unavailability of supplemental systems to assist in classifying defects when the SEM-based image is not amenable to classification.
There exists a need for an arrangement that is capable of quickly and accurately redetecting defects on materials such as semiconductor wafers. There also exists a need for an arrangement that is capable of classifying defects that are not detectable by an SEM.
These and other needs are addressed by the present invention wherein a defect review system includes both an optical microscope and an SEM, thereby allowing quick and accurate redetection and classification of anomalies such as defects. Specifically, the optical system can be used to redetect a defect reported in a defect map. The optical system can also be used to obtain a highly magnified images of the defects in cases where the SEM cannot obtain an image (e.g., defect buried in a transparent dielectric layer).
In accordance with one aspect of the present invention, an apparatus is provided for reviewing defects on an object""s surface, based on a previously generated defect map. The apparatus comprises a stage, an optical microscope, an imaging unit, a particle beam imaging system, and a translation system. The stage functions as a platform upon which the object may be placed. The optical microscope includes an illumination source that directs a beam of light toward a selected portion of the object surface along an illumination path. The optical microscope is used to redetect the defects on the object surface based on information contained in the defect map. The imaging unit is coupled to the optical microscope and generates an image of the selected portion of the object surface. The particle beam imaging system converges a beam of particles, along a prescribed axis, to a focal point. The translation system moves the stage a predetermined displacement such that the redetected defect will be positioned proximate the focal point. The defect can then be reviewed by the particle beam imaging system.
According to one specific embodiment of the present invention, the optical microscope is configured to selectively provide different types of illumination. For example, the selected portion of the wafer can be examined under a bright field illumination, dark field illumination, or both. Hence, defect redetection can be improved by utilizing an illumination type which is most similar to the illumination type used by the inspection microscope to construct the defect map.
According to another specific embodiment of the present invention, an optical redetection and review system is configured together with an optical focusing system. Specific optics are provided to allow automatic autofocusing, optical redetection, and optical review.
In accordance with another aspect of the present invention, a method for reviewing defects on an object surface, based on a previously generated defect map, comprises the steps: viewing selected portions of the object surface, based on coordinates from a defect map, to redetect the defects; determining stage coordinates corresponding to the location of the redetected defects; moving the object surface to position the redetected defects proximate a focal point of a particle beam imaging system; and reviewing the redetected defects using the particle beam imaging system.
Additional advantages and novel features of the present invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.